In backend call: create document - user record reference document id not available?

I am getting the following bug:

I used the "social network template" and changed it quite a lot, including changing the database (added attributes, changed the name from Users to User, etc.). In the original, it doesn't show this problem:

As you may notice, the difference is that in my code, it shows "User Record Reference Document Id" while in the original there is no mention of "document id". Why is this happening? How can I fix it?